Alloy design concept for the development of a new class of austenitic heat resistant
steels strengthened by Fe2M Laves phases (M: transition metals) has been proposed. The phase
diagram studies on Fe-Ni-M ternary systems demonstrate that Fe2Nb with C14 structure is the most
promising, because more than 40at% Ni can dissolve into the Fe sublattice sites and large γ+Fe2Nb
two-phase region exists along the equi-niobium concentration direction. The control of the c/a ratio
of the Laves phase using the composition homogeneity region by alloying makes it possible to
disperse the Laves phase finely in the γ matrix. Based on the knowledge, a model alloy Fe-20Cr-
30Ni-2Nb (at%) was proposed and the TTP diagram of the Laves phase was constructed. The Laves
phase homogeneously nucleates in the matrix and its fine morphology remains almost unchanged
even after long-term aging at 1073K.